1. Field of the Invention
The present invention relates to gas turbine engines and particularly to vibration damping of turbine rotor blades and controlling the temperature of blade attachment structures.
2. Description of Related Art
Gas turbine engines include turbine sections comprising a plurality of blades or buckets mounted to the periphery of a rotor wheel or disk in angularly spaced relation. The turbine blades of typically plural rotor disk stages project into an axially flowing hot gas stream to convert the kinetic energy of this working fluid to rotational mechanical energy. To accommodate material growth and shrinkage due to variations in temperature and centrifugal forces, the blades are typically provided with roots of a "fir tree" configuration, which are captured in dovetail slots in the rotor disk periphery. During engine operation, vibrations are induced in the turbine buckets. If left unchecked, these vibrations can result in premature fatigue failures in the blades.
To dissipate the energy of these vibrations and hence lower vibrational amplitude and associated stresses, it is common practice to dispose dampers between the blades and the disk or between adjacent blades in positions to act against surfaces of tangentially projecting blade platforms. When the turbine disk rotates, the dampers are pressed against the platform surfaces by centrifugal forces. As the blades vibrate, the damper and platform surfaces slide on each other to produce frictional forces effective in substantially absorbing and thus dissipating much of the vibrational energy.
In addition to vibration damping, another important consideration is controlling the temperature of the blades. The airfoil portion of the blades are directly subjected to the high temperature working fluid and thus are typically cooled by air tapped from an upstream compressor and channelled through internal blade passages. It is also important that the platforms, roots and shanks blade attachment structure (blade and disk posts) not overheat. The blade platforms serve as shrouds defining the radially inner bounds of the working fluid annular flow path through the turbine section and thus are also directly subjected to the high temperatures of the working fluid. However, there are necessarily axially extending gaps between the platforms of adjacent blades through which the hot working fluid can be ingested radially into the interblade cavities beneath the platforms. It is known to utilize vibration dampers that assume damping positions against the undersides of the platforms spanning these axially extending gaps to block the radial flow of working fluid into the interblade cavities.
These known damper/seals are not however effective in sealing the circumferentially extending gaps at the forward (upstream) and aft (downstream) edges of the platforms. Even with the addition of forward and aft seals, it is found that working fluid leaks past these seals to produce an axial flow of high temperature gases washing through the interblade cavities under the motivation of the working fluid pressure differential existing between the upstream and downstream sides of the blades. This "in-wash" of working fluid has a compound effect on the temperature of the blade attachment structure. The working fluid heats the blade platforms and shanks by convection, which then become thermal radiators and conductors propagating heat to the blade roots and the disk posts. Excessive heat coupled with the mechanical loadings experienced under turbine operating conditions can lead to premature material failure of the blade attachment structure.